3 Answers

Venusaurite:10,000-1,000,000 (If Bulbasaur was not chosen)Charizardite X/Y:10,000-1,000,000 (If Charmander was not chosen)Blastoisite:10,000-1,000,000 *(If Squirtle was not chosen)

The price depends on how much Lumiose Style you have. You can increase your style drastically by buying 99 Premier Balls from the Poke Mart inside the Poke Center.

You must find all of the other Mega Stones either by them being given to you or finding them with the Upgraded Mega Ring (more info here) between 8-9 PM. Locations told here.

As for money, going into the Battle Chateau, writing a Gold Writ of Invitation while making your lead Pokemon hold an Amulet coin gives good money over the course of the day when more trainers come. Battling this with Happy Hour Inkay or Pay Day Persian increases the value of this even more.

You can also battle in Restaurant Le Wow in Lumiose perfectly (following the amount of turns) to get 25 Balm Mushrooms, which sell for 156,250. You can also scavenge for Balm Mushrooms in trashcans in the Pokemon Village.

The only Mega Stones you can buy are the Kanto Starter's Mega Evolution Stones. (Charizardite X/Y, Blastoisite, Venasaurite)

They are available at $1,000,000 at first. Lots, right? Sure is. However, this price can be lowered as you become more stylish. You can click here to find out how you can become more stylish. The price can be decreased so much, it can get to $10,000! So, just increase your style and sooner or later you'll have the Mega Stones. ;)

for money I'd suggest a team with at least 3 level 50+ Pokemon with pick up and sell their items or go to the restaurants in lumiose city and have your lead Pokemon hold the amulet coin and for the mega stones they are available 8PM-9PM in several area some are easy to find while others are quite difficult there are to many for me to list so you can go to this site to find all mega stone locationshttp://www.serebii.net/xy/megaevolutions.shtml